INTRODUCTION. 13 silica from the bottom. Moreover, the amount of ore is not constant nor the rate at which it is added, for on some heats scarcely any ore is thrown in, on others there may be 500 pounds added in three 'or four hours, and on others there may be 3,000 pounds used in the same period of time. As a matter of fact, there is very little difficulty in maintaining a very regular chemical composition if moderate judgment be exercised and the additions of ore are regulated by the temperature of the furnace and the condition of the metal. Many an open-hearth melter has never heard of silica, and yet can keep a constant percentage of it in his slag. This is due to the fact that the slag regulates itself to a great extent. The pig-iron used in the charge always contains silicon and this furnishes silica. If the amount is not sufficient, there will be a cutting away of the sand bottom to supply more. We thus have by the wearing of the bottom an inexhaustible source of supply of silica. In the same way we have a similar supply of iron oxide by the oxidation of the iron of the bath. If iron ore is added, this is the easiest way for the slag to get the oxide, since it simply appropriates it to its own use. Iron ore is a compound of two atoms of iron with three atoms of oxygen, expressed in chemistry thus—Fe203—,wherein Fe is iron and 0 is oxygen, and the figures represent the proportions. If the slag contains too high a percentage of silica, and needs more iron oxide, and if under these conditions iron ore is added, then only one of these atoms of oxygen goes toward oxidizing the silicon and carbon of the bath. This leaves two atoms of iron and two atoms of oxygen, and these unite together to form two parts of a different oxide, FeO, or since there are two atoms of each, thus—2FeO. The extra atom of oxygen has united with carbon and formed a gas in which one atom of carbon unites with one atom of oxygen. In chemistry this action is expressed thus: C-}-0=CO. The symbol C stands for carbon, and 0 for oxygen, and when united in equal proportions, they form CO, which is the chemical symbol for carbonic oxide. "The whole operation of adding iron ore to an open-hearth bath, when only the extra atom of oxygen is given to the carbon, and the rest of. the oxide stays with the slag, may be expressed by the following simple chemical formula: Fe2Q3+C=2FeO+CO.